One important aim in the development of power semiconductor devices, such as power transistors or power diodes, is to produce devices with a high voltage blocking capability but, nevertheless, a low on-resistance (RON).
Power semiconductor devices, such as power transistors or power diodes, usually include a drift region that mainly defines the voltage blocking capability and the on-resistance of the semiconductor device. In a power transistor, such as a power MOSFET or a power IGBT, the drift region is arranged between a body region and a drain region and is doped lower than the drain region. In a power diode (where the drift region is also referred to as base region) the drift region is arranged between a p-emitter and an n-emitter and has a lower effective doping concentration than each of the two emitter regions.
The on-resistance of a conventional power transistor is dependent on the length of the drift region in a current flow direction and on the effective doping concentration of the drift region, wherein the on-resistance decreases when the length of the drift region is reduced or when the effective doping concentration in the drift region is increased. In a diode or an IGBT, the voltage drop across the drift region when the diode or the IGBT is forward biased is dependent on the length of the drift region in a current flow direction and on the effective carrier concentration of the drift region, wherein the voltage decreases and, therefore, losses are reduced, when the length of the drift region is reduced or when the effective carrier concentration is increased. When a bipolar device like the diode or the IGBT is forward biased injection of electrons and holes increases the effective carrier concentration of the drift region to above the doping concentration of the drift region. However, in a transistor as well as in a diode, reducing the length of the region or increasing the doping concentration reduces the voltage blocking capability.